Approach to extending life of gas turbine engine

ABSTRACT

Methods of increasing lifetimes of components in a gas turbine engine. Ordinarily, components are replaced after they experience a certain number of thermal cycles, or a certain operating temperature limit is exceeded. Under one form of the invention, the components are not replaced at that time, but are subjected to increased cooling, which decreases the highest temperature reached in subsequent thermal cycles. Also, many components are replaced when acceleration of an engine falls below a target. In one form of the invention, the components are not replaced, but (1) scheduled fuel flow is increased and (2) compressor stall margin also increased, in order to attain the target acceleration.

CROSS REFERENCE TO RELATED APPLICATION(S)

This is a divisional of application Ser. No. 10/142,240 filed on May 09,2002, which is hereby incorporated by reference herein.

TECHNICAL FIELD

The invention relates to gas turbine engines and, particularly, tocontrol systems used in such engines.

BACKGROUND OF THE INVENTION

Components in gas turbine engines are normally replaced when theiruseful lifetimes are reached. Of course, if damage occurs to acomponent, that component will be replaced earlier.

However, in some cases, the lifetimes of components are measured not bythe structural viability of the components, but by other factors. Suchcomponents are replaced, even though they are structurally sound andcould remain in service.

For example, if the acceleration of an engine falls below a target, atype of overhaul is undertaken, wherein numerous components arereplaced. However, in many instances, the slowly accelerating engine isperfectly sound, and contains sound components. The engine merelysuffers from slow acceleration, and the overhaul and componentreplacement are undertaken for that reason.

As another example, some components are replaced after they haveexperienced a certain amount of thermal cycling, or have been exposed tohigh temperatures for certain lengths of time. However, these componentsare not necessarily faulty.

The invention has developed an approach to increasing lifetimes ofcertain components in gas turbine engines.

SUMMARY OF THE INVENTION

In one form of the invention, a particular type of acceleration isdemanded of a gas turbine engine. If the engine's rate of accelerationfails to meet a target, then both (1) scheduled fuel flow and (2)compressor stall margin are increased for subsequent accelerations,thereby causing the acceleration to reach the target.

In another form of the invention, after a component has experienced aspecified number of thermal cycles, cooling to the component isincreased.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic of a generalized gas turbine engine.Block 117 indicates hardware and software which implement variousfeatures of the invention.

FIG. 2 is a flow chart illustrating processes implemented by one form ofthe invention.

FIG. 3 illustrates symbolically an increase in scheduled fuel flow, andis not drawn to scale.

FIG. 4 is a generalized flow chart, in graphical form, illustratingprocesses implemented by one form of the invention.

FIG. 5 is a flow chart of the type shown in FIG. 4, but representing adifferent time-temperature pattern.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified schematic of a gas turbine engine 3. Incoming air4 is compressed by a compressor 6, and the compressed air is deliveredto a combustor 9. Fuel 12 is injected into the combustor 9, and isburned.

The hot combustion products 15 are delivered to a high-pressure turbine18, which extracts energy from the combustion products 15. That energyis returned to the compressor 6, by way of shaft 19, for compressingfurther incoming air. The combustion products 15, now somewhat cooled,are delivered to a low-pressure turbine 21, which extracts furtherenergy, and delivers the energy to a fan 24, by way of shaft 27. The fan24 provides thrust indicated by arrow 30. Gases leaving the low-pressureturbine 21 also provide thrust indicated by arrow 33.

A simplified description of a twin spool turbofan engine has been given.It is emphasized that the invention is not limited to that particulartype of engine, but is applicable to gas turbine engines generally.

In actual use, an operator (not shown) of the engine 3 will demand thatthe engine undertake an acceleration. A control system 50 handles theacceleration. The control system 50 is ordinarily attached to a casingof the engine 3, but is shown as displaced from the engine 3, to avoidclutter.

It is noted that executing the acceleration is not a simple matter ofincreasing the rate of delivery of fuel 12 to the combustor 9. Rather,the delivery of fuel 12 is scheduled by the control system 50, and theschedule is adjusted in an upward direction.

Scheduling is a term-of-art. Scheduling refers to the fact that theamount of fuel delivered, in pounds per second, is progressively changedas various measured parameters in the engine change during theacceleration. In addition, the control system 50 may adjust otherparameters, or structures, within the engine 3 during the acceleration.

For example, inlet guide vanes, not shown but known in the art, adjustthe angle of attack of incoming air to the individual blades (not shown)within the compressor 6. As compressor speed increases, as occurs duringthe acceleration, the control system 50 adjusts the inlet guide vanes tomaintain a proper angle of attack.

Thus, the delivery of fuel to the engine 3 during an acceleration isscheduled by the control system 50, and the scheduling may beaccompanied by adjustment of certain mechanisms within the engine, suchas inlet guide vanes. Scheduling of fuel during an acceleration is knownin the art.

As stated in the Background of the Invention, over time, the rate ofacceleration of engine 3 can diminish. That is, an engine having 2,000hours of use may accelerate slower than a new engine. Under one form ofthe invention, this reduction in acceleration is mitigated by thefollowing strategems.

First, a determination is made as to whether the acceleration fallsbelow a target. This determination can be made, for example, bydemanding take-off acceleration from an engine used in an aircraft.Block 100 in FIG. 2 represents this process. As stated above, controlsystem 50 schedules fuel to the engine during this acceleration.

Next, actual acceleration of the engine is measured. That is, the timerequired for the rotational speed of the engine to reach full power ismeasured. Block 105 in FIG. 2 indicates this process.

Next, a determination is made of whether the acceleration meets atarget. Block 110 in FIG. 2 indicates this process. The determinationcan inquire, for example, whether full speed was reached in a specifiednumber of seconds. The value of full speed, in rpm, as well as thenumber of seconds required to reach that value will vary, depending onengine type.

Next, if the acceleration failed to meet the target, the inventionadjusts the fuel schedule, as by increasing the rate of delivery by 5percent. Block 115 in FIG. 2 indicates this process. Also, plot 125 inFIG. 3 indicates the process graphically. Assume that the engine 3, whennew, used the simplified fuel schedule 120 indicated. If the proceduresof FIG. 2 indicated that the fuel schedule 120 should be modified, thenthe schedule 120 can be raised by 5 percent, to schedule 125, asindicated in FIG. 3.

Raising fuel flow, as just described, can decrease stall margin of thecompressor 6. Block 140 in FIG. 2 indicates that stall margin isincreased, in order to counteract the decrease. One approach toincreasing stall margin is to increase compressor bleed. Block 145 inFIG. 1 indicates a compressor bleed. Compressor bleed, by itself, isknown in the art, as is its actuation.

When the acceleration terminates, as when full speed has been reached,the compressor bleed is terminated. Other compressor bleeds, unrelatedto the invention, may still be maintained.

In a pre-existing engine, the invention can be implemented usingexisting bleed valves, such as anti-icing valves or transient valves canbe opened. In a newly designed engine, dedicated valves for the purposeof block 140 can be implemented.

Therefore, as thus far described, one form of the invention ascertainswhether the acceleration of the engine has deteriorated beyond a targetvalue. If so, scheduled fuel flow is incremented upward, and compressorbleed during the acceleration is increased.

In this particular example, acceleration of the engine was actuallymeasured, in order to ascertain whether it meets the required target. Inother situations, the acceleration available in the engine can beinferred, or estimated, without actual measurement. For example, a fleetof engines can be monitored, and it may be determined that, after Xtake-off cycles in the average engine, acceleration deteriorates to theextent that the target is not met. Accordingly, it would be determinedthat whenever an engine of similar type experiences X take-off cycles,irrespective of actual acceleration performance, the fuel scheduleshould be adjusted as in block 115 in FIG. 2.

As a more complex example, well-developed computer models of gas turbineengines exist. These models allow computation of behavior of the engine,such as acceleration, from measurement of other parameters. For example,an engine designer knows what particular ensemble of parameters, such astemperatures, pressures, fuel flow, and so on, to measure in an actualengine, in order to allow the computer model to predict the accelerationof that engine.

As a related example, an ensemble of engine parameters can be measuredin a fleet of engines. The acceleration of each engine can be measured.A correlation between the parameters and the accelerations can beobtained. This correlation allows one to measure the correspondingensemble of parameters in an engine of similar type, and thereby predictthe acceleration of that engine.

Once a determination is made that a given engine has experiencedsufficiently reduced acceleration, the correctives, or correctivemeasures, of blocks 115 and 140 in FIG. 2 are taken. These correctivescan be taken in at least two different ways.

In one approach, a test acceleration is undertaken. Block 117 in FIG. 1indicates software and hardware which performs the test. If the testindicates that correctives are required, then the fuel schedule isadjusted, as indicated in FIG. 3, and the adjusted fuel schedule is usedfor future accelerations. Also, the increase in compressor stall marginis undertaken in subsequent accelerations.

Restated, the correctives are not undertaken in the test acceleration.Of course, if the test acceleration takes the form of a computersimulation, or other simulation, the correctives would not be undertakenat that time either, but in future actual accelerations.

In a second approach, the correctives are applied in real-time, as theacceleration is executed. For example, the inquiry of block 110 in FIG.2 would be undertaken, for example, during the first two seconds of anacceleration. If correctives are deemed necessary, then fuel flow andcompressor stall margin are both increased during the remainder of theacceleration.

Once the procedures of FIG. 2 are executed, they may be repeated, toassure that the resulting alteration in fuel flow is sufficient toattain the desired acceleration. If not, in each repetition, thecorrectives are applied, and the repetitions continue until the desiredacceleration is reached.

A limit may be placed on the repetitions. The limit may be measured by(1) number of repetitions, such as five, (2) a maximum limit on fuel, interms of pounds per hour, or (3) a maximum limit on compressor bleed, interms of pounds per hour. If the limit is reached, and the desiredacceleration has not been reached, then the engine is overhauled, orotherwise serviced, in the usual manner.

A distinction should be made between the process of increasing scheduledfuel flow, and a prior art process which may seem to be related, but isnot. As to the latter, a driver of a car may attempt to pass anothercar. During the passing maneuver, the driver may see that the maneuverneeds to be executed more quickly than originally estimated, and willpress the accelerator pedal, to attain greater acceleration. That is,the driver (1) sees that existing acceleration is insufficient and (2)increases fuel delivery until a higher acceleration is attained.

However, the two steps undertaken by the driver are not found in theinvention. One reason is that the invention increases scheduled fuelflow. That is, actual fuel flow is not under the direct control of thepilot of an aircraft powered by gas turbine engines. There is no trueanalogy between the accelerator pedal of the previous paragraph and thethrottle lever in a jet aircraft.

From another perspective, scheduled fuel flow, in effect, is anequation. It may be represented asFuel Flow=AX1+BX2+CX3+DX4 and so on,wherein fuel flow is measured in pounds per hour; X1, X2, X3, and X4 aremeasured operating parameters of the engine, such as rpm, stator vaneangle, and so on; and A, B, C, and D are weights or constants.

One approach to increasing scheduled fuel flow under the inventionwould, in concept, be to increase the constants A, B, C, and D by thefive percent stated above, but only during accelerations.

In another form of the invention, replacement of components in an engineis deferred by another stratagem. Many components are replaced whentheir thermal history reaches a limit. For example, a cycle is oftendefined as an excursion from a low temperature T1 to a high temperatureT2. Temperature T1 is often taken as a standard nominal ambienttemperature, and is the temperature of the component when cold, when theengine is dormant. When the component experiences a given number ofcycles, the component is replaced.

Under the invention, the component is not replaced at that time, butincreased cooling is applied to the component. One effect of theincreased cooling can be to prevent the component from reaching thehigher temperature T2. As the component experiences further thermalcycles, although not to the original higher temperature T2, furtheradditional cooling is applied.

FIG. 4 is a generalized graphical flow chart indicating one mode ofoperation of the invention. During time interval 200, the engineoperates in the usual manner, and provides a standard amount of cooling.Thus, at this time, no additional cooling load is placed on the engine,so that no penalties on thrust or specific fuel consumption areincurred.

At time 205, a determination is made, as by sensing exhaust gastemperature using the control 50 of FIG. 1 or otherwise, that thecomponent in question has reached a first threshold of thermal cycling.

In response, the control 50 in FIG. 1, or other apparatus, increasescooling of the component. The cooling can be increased by increasing acompressor bleed. Such increases are known in the art. The component inquestion can take the form of a turbine blade, a turbine shroud, or anycomponent generally subject to thermal cycling.

The increase in cooling is indicated by the inflection point 210 in FIG.4. After the increase, the component then experiences the thermalexcursions generally indicated as 215. The higher temperature T3 towhich the component is subject has been reduced.

This approach may be repeated, as indicated by inflection points 220,225, and so on. That is, after a second threshold of cycles has beenreached, as at point 230, cooling may be further increased, as indicatedby inflection point 220. The component experiences thermal excursionsbetween T1 and T4.

Again, after a third threshold has been reached, as at point 235,cooling may be further increased, as indicated by inflection point 225.The component experiences thermal excursions between T1 and T5, and soon.

In the example just given, the increase in cooling was prompted byattainment of a given number of thermal cycles by the component inquestion. Other conditions can be used to initiate the increase incooling, such as (1) attainment of a given number of thermal cycles byanother related component, (2) attainment of a given number of enginetake-off and landing cycles, (3) attainment of a given number of peaktemperatures by the component, (4) attainment of a given number of hoursof engine operation, or (5) other conditions. The generalized criterionwhich initiates the increased cooling is whether the component, orengine, has reached a predetermined degree of deterioration, as thatterm is known in the art.

FIG. 5 is similar to FIG. 4, but illustrates another mode of temperaturevariation which may occur. During time period 250, the temperature of acomponent initially experiences an upward excursion from a basetemperature T10. However, that base temperature increases, as indicatedby envelope 253.

At time 265, the peak excursion reaches temperature T12, which is nearor at a limit as indicated, the amount of cooling is increased. Thiscauses the temperature excursions to follow the pattern shown in timeperiod 255.

One feature of the invention is that compensating actions are takenwhich tend to maintain compressor stall margin at a constant value. Forexample, the increase in scheduled fuel flow, such as that indicated bythe increase in schedule 125 in FIG. 2, can decrease compressor stallmargin. The increase in compressor bleed which is subsequently taken, byitself, tends to increase compressor stall margin. However, when coupledwith the increase in scheduled fuel flow, the increase in compressorstall margin reduces the decrease in stall margin caused by the increasein scheduled fuel flow.

Thus, the two actions have opposite effects, and tend to compensate eachother, as respects compressor stall margin.

Numerous substitutions and modifications can be undertaken withoutdeparting from the true spirit and scope of the invention.

1-8. (canceled)
 9. Method of operating a gas turbine engine, comprising:a) demanding an acceleration in the engine, wherein fuel flow isscheduled; b) ascertaining whether rate of acceleration meets a targetand, if not, c) increasing scheduled fuel flow.
 10. Method according toclaim 9, further comprising increasing compressor stall margin ifacceleration fails to meet the target.
 11. Method according to claim 10,wherein compressor stall margin is increased by increasing compressorbleed.
 12. Method according to claim 9, wherein fuel flow and compressorstall margin are increased in subsequent accelerations, but not theacceleration of paragraph (a).
 13. Method according to claim 9, furthercomprising: d) taking other actions which increase compressor stallmargin during said acceleration.
 14. A system, comprising: a) a gasturbine engine in which fuel flow is controlled according to one, ormore, schedules; b) a testing system for ascertaining whether engineacceleration controlled by a given schedule meets a target accelerationand, if not, raising fuel flow in said schedule for subsequentaccelerations.
 15. System according to claim 14, and further comprising:c) a system for reducing decrease in stall margin caused by the raisingof fuel flow in subsequent accelerations if the target is not met.16-25. (canceled)